US President Donald Trump denied on Sunday having asked then FBI director James Comey to stop investigating ex-national security advisor Michael Flynn, who has since pleaded guilty to lying to the FBI about discussions with Russia.

Trump also insisted he and his campaign had not colluded with Moscow in last year's election, and shifted blame on the Justice Department and his Democratic rival Hillary Clinton.

"I never asked Comey to stop investigating Flynn. Just more Fake News covering another Comey lie!" Trump said in a tweet.

But his position was complicated by another Twitter post in which he indicated he had fired Flynn because the national security chief had been untruthful not just to Vice President Mike Pence but to the FBI as well.

That comment appeared to indicate Trump was acknowledging he knew at the time of Flynn's firing in February that he had lied to the bureau's agents.

"If that is true, Mr. President, why did you wait so long to fire Flynn?" asked Representative Adam Schiff of California, the top Democrat on the House Intelligence Committee.

"Why did you fail to act until his lies were publicly exposed? And why did you pressure director Comey to 'let this go?' "

White House officials, however, told The New York Times that Trump was only referencing Flynn's guilty plea for lying to the FBI about his conversations with then Russian ambassador Sergey Kislyak over sanctions former president Barack Obama slapped on Russia for alleged election meddling.

And two people briefed on the matter said the Twitter post was in fact written by Trump's personal lawyer John Dowd, who apologized to the White House for the tactless language.

After he was fired himself in May, Comey testified under oath before a Senate panel that, a day after Flynn's firing, Trump asked him to drop an investigation into the former national security advisor.

A lingering part of the drama has been that after the White House learned through the Justice Department that Flynn lied to the White House about discussing sanctions with the Russian ambassador, Trump still waited 18 days to fire him.

Trump said he had the Russia probe in mind when he fired Comey. The move backfired and led the Justice Department to appoint Robert Mueller as special counsel.

Mueller's focus goes beyond possible collusion with Russia to business dealings and whether Trump himself tried to thwart the investigation.

US media reported that senior FBI counterintelligence official Peter Strzok was removed from the investigation over the summer for sending text messages critical of Trump.